Nitride semiconductors have been recently produced as materials used for a high bright pure green LED and a blue LED in various light sources for a full color LED display, a traffic signal and an image scanner and the like. These LEDs basically have such a stricture that a buffer layer, a n-side contact layer made of Si-doped GaN, an active layer of SQW (Single Quantum Well) made of InGaN or MQW (Multi Quantum Well) including InGaN, a p-side cladding layer made of Mg-doped AlGaN and a p-side contact layer made of Mg-doped GaN are laminated sequentially on the sapphire substrate. Such LEDs show excellent properties and for example, at 20 mA, for blue LED having a light emitting wavelength of 450 nm, 5 mW of output and 9.1% of an external quantum efficiency can be achieved and for green LED having a light emitting wavelength of 520 nm, 3 mW of output and 6.3% of an external quantum efficiency can be achieved.
The inventors have first realized laser emitting of 410 nm at room temperature by using the above nitride materials and reported it in Jpn. J. Appl. Phys. 35 (1996)L74 and Jpn. J. Appl. Phys. 35 (1996)L217. The laser device comprises the DH structure where the active layer is MQW having InGaN well layers and showed the following data:                Threshold current: 610 mA;        Threshold current density: 8.7 kA/m2;        Wavelength: 410 nm        (pulse width 2 μm and pulse cycle 2 ms)        
The inventors have first succeeded in CW (Continuous-Wave) Oscillation or Operation at room temperature and reported it in Gijutsu-Sokuho of Nikkei Electronics issued on Dec. 2, 1996, Appl. Phys. Lett. 69(1996) and Appl. Phys. Lett. 69(1996) 4056.
The laser diode showed a lifetime of 27 hours at 20° C. under the threshold current density of 3.6 kA/cm2, the threshold voltage of 5.5 V and the output of 1.5 mW.
Thus, nitride semiconductors have been produced as materials for a LED. And for a LD, continuous-wave oscillation or operation of as long as a few tens hours can be achieved. However, a further enhancement of the output is required in order to use LEDs for illumination lights, outdoor displays exposed to the direct rays of the sun and the like. And it is necessary to improve LDs, in order to decrease the threshold in LDs and realize a longer lifetime of the LDs and to use the LDs in light sources such as the light-pick-up, DVD and the like. Said LD showed a forward current of 20 mA and a forward voltage (Vf) of near 3.6V. Further decrease of Vf leads to decrease of generation of heat in the device, resulting in increase of reliability. It is extremely important to decrease the threshold voltage in the laser devices to realize a longer lifetime of the devices.
In view of such circumstances, this invention has been accomplished. The main object of the present invention is to enhance the output of the nitride semiconductor devices such as LED and LD and to decrease Vf and the threshold voltage thereof, resulting in the enhancement of the reliability of the devices. Particularly, the first object of the present invention is to increase the carrier concentration in the n-type contact layer and decrease the resistivity thereof.
Further, the second object of the present invention is to provide an n-type nitride layer structure in which the carrier concentration in the n-type contact layer is increased and the crystallinity of the nitride semiconductor layer formed on the n-type contact layer can be enhanced.